1. Field of the Invention
The invention relates to Quantum Key Distribution.
2. Discussion of the Background
The first quantum key distribution (QKD) protocol was published by Charles Bennett and Gilles Brassard in 1984 and is now known as “BB84.” In 1992 Bennett published a “minimal” QKD scheme (“B92”) and proposed that it could be implemented using single-photon interference with photons propagating for long distances over optical fibers.
U.S. Pat. No. 5,307,410 to Bennett discloses a system for transmitting a cryptographic key information between two entities. The teachings of that patent are incorporated herein by reference.
U.S. Pat. No. 6,188,768 to Bethune et al. discloses another system for transmitting a cryptographic key information between two entities. The teachings of that patent are incorporated herein by reference.
F. J. MacWilliams and N. J. A. Sloane, “The Theory of Error-correcting codes,” North-Holland, 1977, and D. Gottesman's Ph.D. thesis, pp. 8-10, available at the Cornell University Library's ArXiv website in the section “quantum physics” as publication number 9705052, discuss classical and quantum coding theory. The teachings of these publications are incorporated herein by reference.
A qubit is a mathematical representation of the wave function of a two level quantum mechanical system.
A Quantum Key (QK) is a series of digital values (or more generally a series of values in an arbitrary base) derived from transmission of information in a Quantum Key Distribution (QKD) system.
QKD means the transmission of information from a sender to a receiver via a signal strength low enough so that quantum mechanical effects are significant wherein the information encodes a QK. In particular, QKD refers to the transmission of information in which a statistical error rate in reception of a series of transmitting datum is necessarily significantly affected by any measurement of the transmission between the sender and the receiver.
A QKD system is a system providing the means for performing QKD.
An autocompensating QKD system means a system in which two pulses are used to null out effects of the transmission medium on properties of the pulse in which information is encoded. Bethune et al. column 4 lines 25 to 35 disclose an autocompensating QKD system.
Reference herein to numbers of photons per pulse means the average number of photons per pulse unless context indicates otherwise, such as by the use of the word actual to characterize a pulse.
A single photon pulse as used herein has the same meaning ascribed to it at Bethune et al. column 5 line 61 to column 6 line 5, which pulses that each contain no more than one, and on average significantly less than one photon present in each pulse.
A multi photon pulse as used herein means the average number of photons in a set of pulses, in which each actual pulse may contain more than one photon, and in which set there are a significant fraction of the actual pulses containing no more than one photon. In this context, the significant portion means enough pulses containing no more than one photon to ensure that a resulting QK is secure. Thus, the significant portion at the receiver may be for example any one of 1, 10, 20, 30, 40, 50, 60, 70, 80, or 90 percent, depending upon the algorithm used to remove errors from the final QK, the error rate, and the number of qubits of information actually transmitted from the sender.
QKD systems may result in two parties using the system having similar but not identical sets of key values, such as digital values, or sets if bits, for each of their QKs.
Error as used herein refers to those bits for which the QK of the two parties have different values.
Here, by generalized error syndrome, we mean some general information about a quantum key. In other words, an error syndrome is generated by some prescribed logical operations on a quantum key. An example of an error syndrome is the standard error syndrome in classical and quantum coding theory. See, for example, MacWilliams and Sloane, p. 16 and Gottesman, p. 8. It gives the symptom of errors in a transmission or storage of information. For a linear code, any of its codewords, x, must satisfy an equation H x=0, where H is called a parity check matrix and H x denotes matrix multiplication. Now, suppose x is transmitted, but the receiver obtains y. Then, the receiver can compute the error syndrome s=H y, by applying the parity check matrix H on the vector y. The Cascade protocol by Brassard et al provides another example of error syndrome. In Cascade, an error syndrome is some parity bit of a key.
Computer as used herein includes any digital or analog computational device implemented using electronic, optical, or quantum signals or logic devices.
The present inventor recognized that security can be guaranteed by using the novel procedures for QK error detection, correction, and privacy amplification disclosed herein.